Various types of micromechanical components operate best in an open area cavity. In particular, portions of MEMS devices, such as portions of MEMS resonators should be suspended in air within a device to maximize energy reflection between the MEMS resonator and air. In order to suspend a portion of a MEMS device in air, during fabrication, MEMS devices are formed over a sacrificial material, which is subsequently removed, thereby providing the MEMS device portion with free space and allowing movement. However, the sacrificial material is not removed in a controlled manner beneath the MEMS device. Thus, a release area formed where the sacrificial material was removed becomes too large. As such, the sacrificial material disposed under portions of the MEMS device, which do not require free space, such as anchors, is also removed. However, when the sacrificial material disposed under the anchors is removed, performance and stability of the MEMS device degrades, thereby degrading overall performance and reliability of the MEMS device.
In an effort to control the release area, one solution involves forming a cavity, which will provide an open area cavity and then forming the MEMS device over the cavity, as disclosed in U.S. Patent Application Publication No. 2007/0200458 A1 to Yoshino et al. In particular, a bottom electrode and a cavity are initially formed on a device. A piezoelectric substrate is then bonded to the device where the cavity is preformed and the piezoelectric device is thinned down. However, the piezoelectric substrate may warp at the region where the cavity is formed prior to thinning down. Furthermore, thinning down the piezoelectric substrate increases warping. Thus, performance of a device employing the warped piezoelectric substrate dramatically degrades, thereby making a device manufactured according to this method undesirable.
Accordingly, a need exists for a method of forming a MEMS device over a cavity while minimizing warping and controlling a release area of a sacrificial material used to form the cavity such that the cavity forms immediately below a resonator of the MEMS device. Moreover, a need exists for a method of forming a MEMS device having improved performance and improved reliability.